1. Field of the Invention
The present invention relates to an image forming apparatus that prints color material such as toner or the like on a paper sheet.
2. Description of the Related Art
With an image forming apparatus in which an electrophotographic method is adopted, an image carrier called a photosensitive body is charged by a primary charger, and a latent image is formed on the image carrier by an optical system such as a laser or the like. The latent image is developed by a development unit having color material called toner on the image carrier using the toner. Toner that has been developed and held on the image carrier is transferred on a paper sheet by one or two transfers, and fixed to the paper sheet by a thermal fixing apparatus. In a basic printing process of an electrophotographic method, since the above-mentioned electrostatic development is used, the process is easily influenced by temperature and humidity. Therefore, a lot of techniques for stabilizing the printing process have been proposed in order to solve the above-mentioned unstability. For example, Japanese Patent Laid-Open No. 11-75067 proposes a stabilization technique in an electrophotographic image forming apparatus having automatic gradation correction.
With automatic gradation correction, a plurality of pattern images are output on a paper sheet, the pattern images are read with a RGB color reader, and pieces of RGB luminance information that are in a complementary color relationship (red→cyan, green→magenta, blue→yellow, green→black) are extracted. The extracted luminance information is converted into density information with a LUT (lookup table, hereinafter, referred to as “luminance/density conversion table”) in which the relationships between luminance and density that are held in advance is expressed. The density conversion performed for each pattern image, a printing condition for obtaining the desired density is calculated, and the printing condition is registered as the printing condition for subsequent printing. The above-mentioned density is influenced by the uneven surface and spectral characteristics (whiteness and the like) of a paper sheet. Therefore, even if the maximum density is adjusted, the maximum amount of color material will not necessarily be a prescribed amount. Japanese Patent Laid-Open No. 2007-27211 proposes a technique for controlling the density to be a prescribed value and color material to be a prescribed amount by executing CAL (calibration) with use of CAL-dedicated media in which whiteness and unevenness are managed.
However, there are the following problems in the conventional techniques. With a conventional method for determining the maximum amount of color material using density, since the resolution particularly in high density portions is insufficient due to a relationship between the spectral reflectance of color material and the spectral sensitivity of a reader, the precision is not sufficient. As shown in reference numeral 1101 of FIG. 11, even if the amount of color material changes, spectral characteristics in the maximum density portion have change little in the detected spectral wavelength ranges, and density values vary due to lifting of a test chart or influence of various types of noises, and thus it has not been possible to grasp the accurate amount of color material. As a result, variation occurs in the result of correcting the maximum amount of color material, leading to an increase in the amount of color material, and spattering, poor granularity, and poor fixation of color material.